This invention relates to a process for the continuous production of aluminum comprising, in combination, the carbochlorination of alumina in a molten salt bath and electrolysis of the anhydrous aluminum chloride obtained in the bath emanating from carbochlorination.
It has been known for some time that aluminum chloride can be obtained by the carbochlorination of an aluminous ore or even by the carbochlorination of alumina obtained by extraction from an alumina-containing ore. The economic importance attributed to the industrial production of aluminum chloride for catalytic applications or for the electrolytic production of aluminum has prompted experts to conduct detailed research in this field. Thus, numerous processes have been described in the literature for the production of anhydrous aluminum chloride by the carbochlorination of alumina in molten salt baths.
One such process is described for example in French Pat. No. 2,334,625 and comprises contacting alumina with a source of chlorine in the presence of a reducing agent, such as carbon, in a molten salt bath consisting of at least one alkali metal and/or alkaline-earth metal chloride and aluminum chloride, the anhydrous aluminum chloride being collected in gaseous form at the bath outlet.
However, this type of process, although producing anhydrous aluminum chloride sufficiently pure for the electrolytic production of aluminum, is not effective enough to give an hourly yield of aluminum chloride which experts would regard as entirely satisfactory. This is because the alumina is introduced into the molten salt bath in the most common form, i.e. in the form of a fine white powder, the reducing agent, such as carbon, being reduced to a similar particle size.
The chlorine intended for carbochlorination of the alumina is then continuously injected in a stoichiometric proportion into the molten salt bath in which are immersed gas diffusors of known type, such as for example quartz rings, these diffusors giving rise to the formation of a very large number of very small gas bubbles which come into contact with the individual particles of alumina and carbon suspended in the stirred bath. Despite the presence of these diffusors for the gas phase, only a fraction of the chlorine introduced into the bath reacts with the alumina and the carbon while the other fraction is removed from the carbochlorination reactor with the vaporized aluminum chloride.
This lack of thorough contact between the solid and gaseous materials is the cause of a relatively low hourly output of anhydrous aluminum chloride per cubic meter of bath in spite of all the attempts to the contrary which have hitherto been made.
It has also been known for some time, as can be seen from the literature, that aluminum can be produced by the electrolysis of aluminum chloride dissolved in a molten electrolyte consisting of at least one alkali metal halide which is more difficult to reduce than the aluminum chloride itself.
The extensive literature available in this field is above all the consequence of certain observations by experts on the advantages which a process such as this would have over the Hall-Heroult process such as, for example, lower energy consumption, lower consumption of the electrodes by oxidation of their constituent graphite under the effect of the oxygen released during electrolysis of the alumina and, finally, the fact that electrolysis is carried out at a lower temperature.
However, from the time of the first experiments with the electrolysis of aluminum chloride, major disadvantages have always been encountered at an early stage, resulting in the postponement of operation of such a process on an industrial scale. Thus, for example, experts have been confronted by particularly troublesome phenomena because the most significant disadvantages emanate from the presence of metal oxides dissolved or undissolved in the electrolytic bath, such as alumina, silica, titanium oxide and iron oxide.
The reason for this is that the undissolved metal oxides are the cause of a gradual accumulation on the cathodes of a viscous layer of finely divided solids, liquid components of the bath and droplets of molten aluminum which interfere with access to the cathodes of the electrolysis bath and which can result in disturbances to the normal cathode mechanism, i.e. reduction of the cations containing the metal to be produced in various stages of oxidation. Thus, the aluminum chloride present in the viscous layer and consumed by electrolysis is increasingly more difficult to renew and, accordingly, the other chlorides making up the molten salt bath, such as the alkali and/or alkaline-earth metal chlorides, can be electrolysed, resulting in a loss of effeciency of the electricity used and in pollution of the metal produced.
In addition, because the alkali metal chlorides present in the viscous layer, such as the chlorides of sodium, potassium or lithium, are partially electrolysed by the lack of renewal of the aluminum chloride in the proximity of the cathode, they lead to the corresponding metals which infiltrate under cathode potential into the constituent graphite of the electrodes, resulting in their disintegration and destruction. This premature destruction of the cathodes results in the introduction of graphite particles into the bath which contribute to the formation of sludge, causing a reduction in the output of the electrolysis process.
Finally, another equally serious disadvandage, attributable to the presence in the bath of dissolved metal oxides, such as alumina, lies in the release of the anode of oxygen which consumes the graphite. This consumption of graphite interferes with the electrolysis process because it alters the geometric characteristics of the anodes, particularly the anode-cathode interval.
In order to eliminate the above-mentioned disadvantages emanating essentially from the presence of metal oxides and/or oxidized compounds entering the electrolysis bath with the constituents thereof or even being formed in situ due to the infiltration into the electrolysis cell of moisture reacting with the metal formed and with the chloride bath, it was proposed in French Pat. No. 2,158,238 to carry out the electrolysis of aluminum chloride dissolved in a bath of molten salts by a process in which the aluminum chloride content of the electrolysis bath is kept between 1 and 15% by weight by introducing the aluminum chloride continuously or periodically to replace the electrolysed aluminum chloride, limiting the presence of metal oxides and/or oxidized compounds in this bath so that their percentage by weight does not exceed 0.25%. To this end, it is specified that the aluminum chloride introduced into the bath should contain less than 0.25% by weight of metal oxides and that, in particular, the residual moisture which can be introduced by the aluminum chloride itself or which may initially be present in the electrolysis cell should be reduced to a minimum.
In spite of the elaborate precautions which have been taken for handling and using the aluminum chloride, i.e. between the time it is removed in gaseous form from the carbochlorination reactor and the time it is introduced in condensed form into the electrolysis cell, it has been found that this process, although alleviating the above-mentioned disadvantages, does not eliminate them completely.
Aware of the interest which a well-adapted process for the electrolysis of aluminum would have among experts, but equally aware of the disadvantages attending the processes previously described in this field, applicants-continuing their research-have found and perfected a process for the electrolysis of aluminum chloride substantially free from the disadvantages mentioned above, in which the aluminum chloride, which is known to be highly hygroscopic, is not subjected to any harmful manipulation which might result in the presence of residual oxidized compounds in the electrolysis bath.